Document PHP version requirement

doc/02-Installation.md

@@ -14,7 +14,10 @@ chapter.
 
 * [Icinga 2](https://www.icinga.com/products/icinga-2/) with the IDO database backend (MySQL or PostgreSQL)
 * A web server, e.g. Apache or Nginx
-* PHP >= 5.3.2 with the following modules installed: cURL, gettext, intl, mbstring, OpenSSL and xml
+* PHP version >= 5.6.0. Though Icinga Web 2 version 2.5.x is compatible with PHP version >= 5.3.2 there are modules
+  which require a higher PHP version. Also, future versions of Icinga Web 2 will drop support for outdated PHP versions.
+  Packages for Icinga Web 2 already require PHP >= 5.6.0
+* The following PHP modules must be installed: cURL, gettext, intl, mbstring, OpenSSL and xml
 * Default time zone configured for PHP in the php.ini file
 * LDAP PHP library when using Active Directory or LDAP for authentication
 * MySQL or PostgreSQL PHP libraries